Mercruiser 7.4L Overheating

[Scott Strom]

I have a 1997 Wellcraft Martinque with twin Closed Cooled 7.4L Mercruisers with V-Drive and one is overheating when under power. At under 2500 RPM it is fine, but when I try to get on it, it will shoot up to 200 and the other engine runs at a cool 155-160F. If I back off it will cool down. So far I have replaced most everything... Replaced heat exchanger, raw water pump impeller, thermostat, exhaust manifold on the hot side, checked riser, removed and cleaned trans cooler, checked the sea strainers, removed raw water hose into the riser and started the engine and it has good flow. I attached a garden hose to the same hose and turned it on and I had good flow out the exhaust on the side of the boat. The exhaust tube coming out of the riser on the cool side measures 103 degrees F. The hot side measures up to 180F at an idle. There is no water in the oil or smoke. Had a guy tell me it could be the Carb, but the engine purrs like a kitten. I am stumped... HELP!!

 

[Scott Danforth]

welcome aboard

half systems or full systems?

how scored up is your raw water pump housing? have you put a clear hose from the pump to the HX and looked for bubbles in the stream?

you have a flow restriction or air leak somewhere.

if your timing is off, you could run a tad bit hot. however shouldnt be too much higher than normal.

 

[tpenfield]

:welcome: to iBoats . . .

I assume thru-hull raw water pick-ups given the V-drives (right?) Any marine life growth in the pick-up? I've been there/done that with my outdrives . . . a few critters grew inside of the water duct and blocked the water flow.

I chased the problem for months, until I finally pulled the boat and removed the outdrive.

 

[Scott Strom]

I do have thru-hull raw water pickups. We have a lot of weeds at the marina, but I removed the raw water hose that enters the riser and started the engine and it seemed to have good flow. Plus I have good flow of water out the exhaust. Looks the same on both sides of the boat. I had the impeleller changed thinking that may be it and the mechanic said it was ready to change but did not come apart so no loose pieces.